Contextual Advertising
Contextual advertising is where your advertisements are related to your content. So if you have a gadget related blog the advertisements will be related to the gadgets rather than mortgage or financing. If used carefully these can be a very good source of income.
Google Ad sense
You have to sign up with Google for your ad sense account and they will provide you the code to be inserted in your template. This is a very easy method and you can begin showing your ads right away.
You can sign up here.
Chitika
The process is very similar to that of Google. If you have heavy US traffic you can opt for them.
You can sign up here.
Yahoo Publisher network
This one is a real competitor to Google such that Google does not allow displaying the YPN ads along with Google ad sense.
You can sign up here.
